1. Field of the Invention
The present invention relates to a multiplexer, a demultiplexer and a multiplex communication system, such as an optical multiplex communication system or a radio multiplex communication system.
2. Description of the Related Art
In the conventional optical communication system, for example, an optical multiplexer is used for producing and transmitting a multiplexed optical pulse train signal of very high speed, e.g. no less than 10 Gb/s, and an optical demultiplexer is used for demultiplexing the received multiplexed optical pulse train signal.
FIG. 5 shows an example of such a conventional optical multiplex communication system. The system includes an optical multiplexer 300 and an optical demultiplexer 400.
In the optical multiplexer 300, a basic optical pulse train or carrier with a period T, produced by an optical pulse source 301, is divided into two pulse trains which are then fed to two separate modulators, i.e. a modulator 302 and a modulator 303, respectively. The modulator 302 applies amplitude shift key modulation to the received pulse train with data 1 to produce a first optical pulse train signal, while the modulator 303 applies amplitude shift key modulation to the received pulse train with data 2 to produce a second optical pulse train signal. The first optical pulse train signal is then delayed by a half period (T/2) by a delay circuit 304 and combined with the second optical pulse train signal by an optical coupler or the like. Namely, the first and second optical pulse train signals are interleaved on a time axis, i.e. time-division multiplexed with each other over an interval of T/2, so that a multiplexed optical pulse train signal is produced and transmitted at a transmission rate twice the original (see (A) and (B) in FIG. 2).
On the other hand, in the optical demultiplexer 400, the multiplexed optical pulse train signal inputted through an optical fiber, an optical exchange or the like is fed to an optical input port of an electric field absorption type optical modulator (hereinafter referred to as “EA modulator”) 405. Simultaneously, the multiplexed optical pulse train signal is also fed to a clock extractor 401 wherein a sine wave electric signal having the same period as the inputted multiplexed optical pulse train signal is produced from the inputted multiplexed signal. The electrical sine wave signal has a phase such that its crests coincide with the centers of pulses of the multiplexed optical pulse train signal (see (B) and (C) in FIG. 2). Then, the electrical sine wave signal is fed to a ½ divider 402 which produces an electrical sine wave signal (RF signal) with a period twice that of the inputted sine wave signal (see (D) in FIG. 2). The RF signal is fed to an EA drive amplifier 404 via an RF phase adjuster 403. The EA drive amplifier 404 adds a DC bias voltage to the RF signal so that a maximum value (including values around it) approximates to a high voltage of 0 [V] (see (E) in FIG. 2). The biased RF signal is then fed to a modulator drive input port of the EA modulator 405. The RF phase adjuster 403 is for correcting a phase difference caused at the clock extractor 401 and the ½ divider 402.
In the EA modulator 405, an optical pulse train signal fed to the optical input port is transmitted or passes therethrough when a high voltage around 0 [V] is inputted to the modulator drive input port, while it is blocked and not fed to an optical output port when a low voltage around some minus voltage is inputted to the modulator drive input port.
As described before, the multiplexed optical pulse train signal inputted to the EA modulator 405 is a signal obtained by time-division multiplexing the first and second optical pulse train signals with a period T/2. On the other hand, the EA modulator 405 transmits the multiplexed optical pulse train signal with a period T in response to the biased RF signal inputted from the EA drive amplifier 404. Accordingly, only one of the first and second optical pulse train signals is separated from the multiplexed signal and fed to the optical output port (see (F) in FIG. 2).
In the foregoing conventional optical multiplex communication system, however, there are problems that it is not possible to judge which of the first and second signals is extracted in the optical demultiplexer 400 and that selection between the first and second signals is not possible in the optical demultiplexer 400.
These problems are not limited to the optical multiplex communication system, but also applied to other multiplex communication systems such as a radio multiplex communication system.